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Amateur Radio Info & Exams - Propagation

Signals on each band travel between stations in somewhat different ways, depending on various factors, discussed here.

VHF and UHF signals travel primarily in what is termed "line of sight", although this does extend somewhat beyond a true optical path. However, unless you are operating between mountains tops, range is typically limited to tens of kilometres, maybe a hundred or so to a well located repeater. Using SSB or CW, and high gain directional antennas, range can extend beyond this.

More correctly, VHF signals leave your antenna, and continue into space, literally "going off on a tangent". The trick is to get some of that signal to return to earth.

On 6 metres, at times signals can refract from the ionosphere, and this can also occur on 2 metres and 1.25 metres irregularly, to the extent one such mechanism is called Sporadic E. 10 metres, and even 12 metre ionosphere propagation is sporadic. The F2 layer also provides VHF DX at times.

The trails of ionised material left by meteors can reflect signals, allowing for short bursts of extended radio propagation. While this occurs on various bands, the examiner believes that 6 metres is the best.

Tropospheric scatter is a system where a high power UHF or microwave signal is fed into a large dish, directed towards the receiving station. A small portion of the signal refracts due to moisture and dust in the atmosphere, reaching the receiving station which consists of several large dishes, spaced some metres apart, for diversity. With a range of over 1000 km, this system has however fallen out of favour, in favour of satellite systems. It is also called "tropo-scatter", or just "tropo".

If one has had the privilege of seeing an aurora, you will notice that it shimmers and flutters. This rapid changing thus exhibits itself if VHF radio signals bound off it, with rapid signal level changes, and distortion. The term "backscatter" applies to this propagation.

How about we put sheets of metal about 10km above the ground, and bounce signals off them? We do! Signals can reflect from either the bodies of large aircraft, or the condensation trails (that's contrails, not chemtrails, the latter being the result of the combustion of Tetrahydrocannabinol*). VHF signals can pass well long paths, such as Sydney to Melbourne, when aircraft are around half-way along the route.

*THC - probably a little unkind to stoners, but you would have to be off your scone to believe 40,000 pilots and ground-crew would poison their own families. And HAARP is nothing more than a system to experiment with the ionosphere, not a weather or mind control device.

Many of these phenomena provide both DX for broadcast FM and TV, and can lead to interference between stations. Some of these can also cause interference between VHF marine bases, and perhaps air-band bases.

On HF bands, there is a generalisation that, the higher the sun, the higher the frequency which should be used. This also varies with time of year, and the 11 year sunspot cycle. For 10 metres to have regular ionospheric support, there typically must be both a period of high sunspot activity, and daylight. At other times 10 metres has similar properties as 27 MHz CB, with only local coverage.

"Gray-line" relates to MF and low HF propagation. It allows the reception of distant MF stations in the early evening and around dawn. An example is interstate high power ABC stations in Australia, and 2ZB on 1035 from NZ.

Rain and fog have little impact on HF and VHF signals, but rain and snow has significant impact on the microwave bands. Given satellite TV distribution, including to many terrestrial transmission sites is also microwave spectrum, to counter this, national broadcasting organisations use large (6 metre) dishes, and in Darwin they have two sites, for "spatial diversity".

Relevant Questions

Actual exam questions, from the published NCVEC Technician pool.

What should you do if another operator reports that your station’s 2 meter signals were strong just a moment ago, but now they are weak or distorted?
A. Change the batteries in your radio to a different type
B. Turn on the CTCSS tone
C. Ask the other operator to adjust his squelch control
D. Try moving a few feet or changing the direction of your antenna if possible, as reflections may be causing multi-path distortion

Try moving, as various factors can cause points where there are "nulls", answer D.

Why might the range of VHF and UHF signals be greater in the winter?
A. Less ionospheric absorption
B. Less absorption by vegetation
C. Less solar activity
D. Less tropospheric absorption

In locations where trees loose their leaves in fall, the reduction in vegetation related signal absorption, answer B. Depending on your location in Australia, this may occur less, a many trees keep their leaves year-round.

What antenna polarization is normally used for long-distance weak-signal CW and SSB contacts using the VHF and UHF bands?
A. Right-hand circular
B. Left-hand circular
C. Horizontal
D. Vertical

This operation is usually uses horizontal antennas, answer C.

What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization?
A. The modulation sidebands might become inverted
B. Signals could be significantly weaker
C. Signals have an echo effect on voices
D. Nothing significant will happen

In this case signals are can be significantly weaker, answer B. (I have seen a figure of -20 dB.)

When using a directional antenna, how might your station be able to access a distant repeater if buildings or obstructions are blocking the direct line of sight path?
A. Change from vertical to horizontal polarization
B. Try to find a path that reflects signals to the repeater
C. Try the long path
D. Increase the antenna SWR

VHF and UHF signals can be reflected off terrain or buildings, especially if they are metallic, answer B. While the Snowy Mountains hydro-electric scheme was being built, they set up plain metal billboards, which were used to bounce microwave signals off.

What term is commonly used to describe the rapid fluttering sound sometimes heard from mobile stations that are moving while transmitting?
A. Flip-flopping
B. Picket fencing
C. Frequency shifting
D. Pulsing

The variation sounds like driving past a picket-fence with your window down, and hearing the variation in echo from your vehicle, answer B. The term is also applied when a moving station experiences such a fluttering while receiving.

A similar audio effect occurs applying the Size 12 while climbing through Galston Gorge, between the rock wall and the heavy square timber posts supporting the mesh.

What type of wave carries radio signals between transmitting and receiving stations?
A. Electromagnetic
B. Electrostatic
C. Surface acoustic
D. Magnetostrictive

These waves are electromagnetic, answer A. The last two are interesting phenomena, maybe worth looking up on Wikipedia.

Which of the following is a likely cause of irregular fading of signals received by ionospheric reflection?
A. Frequency shift due to Faraday rotation
B. Interference from thunderstorms
C. Random combining of signals arriving via different paths
D. Intermodulation distortion

If signals arrive via two or more paths, at some times the signal will add, to give a strong signal, and at other times they will cancel each other out, resulting in a near zero signal, answer C.

Which of the following results from the fact that skip signals refracted from the ionosphere are elliptically polarized?
A. Digital modes are unusable
B. Either vertically or horizontally polarized antennas may be used for transmission or reception
C. FM voice is unusable
D. Both the transmitting and receiving antennas must be of the same polarization

Once the single has refracted from the ionosphere it is possible to receive it using an antenna of either polarisation, and likewise, both vertical and horizontal antennas can be used to transmit with, answer B. Selection of vertical or horizontal antennas depends, for instance, whether you are hoping to work long distances, or take part in a net covering a few hundred kilometres.

What may occur if data signals arrive via multiple paths?
A. Transmission rates can be increased by a factor equal to the number of separate paths observed
B. Transmission rates must be decreased by a factor equal to the number of separate paths observed
C. No significant changes will occur if the signals are transmitting using FM
D. Error rates are likely to increase

Signals arriving at different times may cause errors, answer D.

Which part of the atmosphere enables the propagation of radio signals around the world?
A. The stratosphere
B. The troposphere
C. The ionosphere
D. The magnetosphere

This is the ionosphere, answer C.

How might fog and light rain affect radio range on the 10 meter and 6 meter bands?
A. Fog and rain absorb these wavelength bands
B. Fog and light rain will have little effect on these bands
C. Fog and rain will deflect these signals
D. Fog and rain will increase radio range

This weather has little effect at these frequencies, so B.

What weather condition would decrease range at microwave frequencies?
A. High winds
B. Low barometric pressure
C. Precipitation
D. Colder temperatures

If you have rain on the signal path of a microwave signal, be this between ground based stations, or from space through a rain storm, signals will fade, in the case of satellite TV, to the extent the signal is lost. Snow accumulating on the feed and dish also kills the signal. A mark on answer C will align with the perforation in the marking template.

Especially at 10 GHz (3 cm), signals can be bounced off storms to make longer distance contacts. Meanwhile on 6 metres, South African Amateurs have found that two stations at a safe distance from a lightning storm can reflect signals off it, perhaps from the ionised columns of air following lightning strikes.

Why are direct (not via a repeater) UHF signals rarely heard from stations outside your local coverage area?
A. They are too weak to go very far
B. FCC regulations prohibit them from going more than 50 miles
C. UHF signals are usually not reflected by the ionosphere
D. UHF signals are absorbed by the ionospheric D layer

UHF signals are rarely reflected (or refracted) by the ionosphere, if ever, answer C. Were the last true, satellite systems at UHF would not work.

The FCC can't prohibit a signal going anywhere, but they used to prohibit deliberate CB communications beyond 250 km (155.3 miles). At least they are not like the failed merchant banker*, Turnbull who claimed that "the laws of Australia override the laws of mathematics", in reference to encryption. * Interesting how rhyming slang matches reality for a few current politicians.

Which of the following is an advantage of HF vs VHF and higher frequencies? A. HF antennas are generally smaller
B. HF accommodates wider bandwidth signals
C. Long distance ionospheric propagation is far more common on HF
D. There is less atmospheric interference (static) on HF

Long distance ionospheric propagation is common, even quite reliable, on most HF bands, while "Sporadic E" providing VHF-DX is rare, answer C. While there are huge multi-yagi arrays on VHF, and vast Chinese Academy of Sciences & CSIRO "Aperture Spherical Telescope" is 500 metres across, bigger than Arecibo, A is mostly wrong. D is generally wrong, but VHF can get noisy in summer. Consider each VHF band is wider than all HF Amateur spectrum combined, about half a TV channel worth; and that 30 metres is only 50 kHz wide, the size of 2 regular FM channels.

What is a characteristic of VHF signals received via auroral reflection?
A. Signals from distances of 10,000 or more miles are common
B. The signals exhibit rapid fluctuations of strength and often sound distorted
C. These types of signals occur only during winter nighttime hours
D. These types of signals are generally strongest when your antenna is aimed west

Signals fluctuate rapidly, and may be distorted, answer B. Aurora occurs at both poles, but are only visually observable during darkness, and darkness at night only occurs in winter at the poles, as there is "midnight sun", or at least twilight in summer.

Which of the following propagation types is most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2 meter bands?
A. Backscatter
B. Sporadic E
C. D layer absorption
D. Gray-line propagation

Sporadic E is the only thing relating to these bands, answer B. Backscatter signals are weak and often distorted.

Which of the following effects might cause radio signals to be heard despite obstructions between the transmitting and receiving stations?
A. Knife-edge diffraction
B. Faraday rotation
C. Quantum tunneling
D. Doppler shift

VHF and UHF signals can "trip" over cliff edges and sharp ridges, in a process called "knife-edge diffraction, answer A.

What mode is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?
A. Tropospheric scatter
B. D layer refraction
C. F2 layer refraction
D. Faraday rotation

Tropospheric scatter can provide communication over around 500 km on a regular basis, answer A. F2 is much more occasional, and the other two are silly.

What band is best suited for communicating via meteor scatter?
A. 10 meter band
B. 6 meter band
C. 2 meter band
D. 70 centimeter band

6 metres is best for meteor scatter, reflecting signals off the ionised trail, answer B. This does not rule out other VHF and UHF bands for such operation.

What causes tropospheric ducting?
A. Discharges of lightning during electrical storms
B. Sunspots and solar flares
C. Updrafts from hurricanes and tornadoes
D. Temperature inversions in the atmosphere

Below the ionosphere, affected sunspots and flares, is the troposphere, and this is the layer at which weather, including the temperature inversions occur, giving long range propagation of VHF, UHF, and SHF signals, answer D. These are related to stable atmosphere, and high pressure systems, so low pressure systems and the related storms are wrong.

What is generally the best time for long-distance 10 meter band propagation via the F layer?
A. From dawn to shortly after sunset during periods of high sunspot activity
B. From shortly after sunset to dawn during periods of high sunspot activity
C. From dawn to shortly after sunset during periods of low sunspot activity
D. From shortly after sunset to dawn during periods of low sunspot activity

High sunspot activity means F layer supports the propagation of 10 metres during the day, answer A.

Which of the following bands may provide long distance communications during the peak of the sunspot cycle? A. 6 or 10 meter bands B. 23 centimeter band C. 70 centimeter or 1.25 meter bands D. All of these choices are correct

Sunspots alter the ionosphere, and thus it is the two bands which are in the HF to VHF transition, 10m and 6m, answer A.

Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations?
A. Radio signals move somewhat faster than the speed of light
B. Radio waves are not blocked by dust particles
C. The Earth seems less curved to radio waves than to light
D. Radio waves are blocked by dust particles

The Earth seems less curved to radio waves than to light, answer C.

T3C12 moved up to replace T3C10, asking what the radio horizon is.

On to: Transceivers

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Written by Julian Sortland, VK2YJS & AG6LE, February 2018.

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